Essential oils are flavor and fragrance materials harvested from plant sources. These materials are obtained from approximately one-third of the known plant families, most often by physical means. Peppermint and spearmint are two such essential oils of significant commercial value. Peppermint and spearmint oils typically are generically referred to as mint oil.
The historical use and value of the mint plant for fragrance and medicinal purposes can be traced back to as early as biblical times. The distillation of mint oil from the plants for use in medicines was practiced in Egypt as early as A.D. 410. The literature reports the distilling of mint oil for an eyewash in Japan as early as A.D. 984. Production began in England about 1696, and in the United States, at Ashfield, Mass., in 1812. Production extended to Michigan in 1835, to Indiana in about 1855, and to the Pacific Coast States around the early 1900's. The yield of oil from mints varies greatly with season, cultural conditions, and geographic location. Both peppermint and spearmint production is currently concentrated primarily in 6 states: Washington, Oregon, Idaho, Wisconsin, Indiana and Michigan. The mint oils produced in each of these regions are often similar in composition, but possess very distinct odor and taste characteristics, caused by regional variances in temperature, photoperiod, and light intensity.
In current times the mint plant, both spearmint and peppermint, are grown in large acreage's, primarily in the aforementioned areas in the United States, as well as other parts of the world, some for garnish purposes, but mostly for the oil that is recovered. The resulting oil is used to provide fragrance and flavor for various products. The mint oil is used because of its ability to mask undesirable flavors and odors, as well as to further enhance products such as gum, tooth paste, perfumes, and the like.
The physical means most often used to obtain the essential oil of a plant include extraction, vacuum distillation, steam distillation, and supercritical gas and solvent extraction. The most widely used method is steam distillation, which greatly takes advantage of the volatility associated with a vast number of essential oils.
Once a crude essential oil is obtained, it is often further processed. This is usually done through classic vacuum and steam distillation procedures to concentrate the oil.
Mint essential oils are primarily obtained by steam distillation of the fresh above ground parts of a mature mint plant. American peppermint oil is obtained from Mentha piperita L. Mentha spicata L. produces spearmint oil, native type and Mentha gentiles L., Cardiaca ger. produces spearmint oil, Scotch type. All of these oils are typically obtained in crude form by doing a large steam distillation of the plants after they have been cut and allowed to dry for several days.
Yields are reduced greatly if the plants are too thick in the field. Thick stands prevent full leaf development and cause lower leaves to fall off, owing to the continued dampness and shading. Rough handling when the herb is too dry also causes loss through shattering of leaves, as most of the oil is recovered from the leaves.
The cost of production and distillation of spearmint and peppermint is very expensive; therefore, it is imperative that the maximum oil possible be recovered from every producing plant if the maximum monetary potential is to be achieved.
The process of removing oil from mint consists of passing steam through the cut hay, thus vaporizing the oil. The steam and oil vapors are then conducted through a condenser, and are reduced to water and oil. As they are collected in receiving vessels, the water and oil separate into layers, with the oil floating on the surface. Although distilling equipment has gradually improved, the method for removing oil from the plant has changed very little.
There is still a need for methods to improve the yield of mint oil extracted and to improve the economy and value of mint oil extraction methods.